Dielectric ceramics for high frequency applications have been, in recent years, used widely, for example, for dielectric resonators and dielectric substrates for MIC. This type of dielectric ceramic for high frequencies have been required to provide high specific dielectric constants ∈r and Q values, for the reduction in size.
On the other hand, the dielectric ceramic for high frequencies are disadvantageous in that when tungsten or molybdenum with a high melting point is used for a conductor material, ceramic multilayer substrates particularly have limited high-frequency characteristics due to the high resistivities of these high melting point metals, and moreover, which are expensive. For this reason, the use of low-resistance and inexpensive low-melting-point metals such as Ag and Cu as the conductor material has been requested.
In order to obtain a ceramic sintered body by co-firing of a conductor material and a ceramic material, there is a need to subject the ceramic material to firing at a temperature which is lower than the melting points of these low-melting-point metals.
Therefore, studies have been actively carried out on ceramic materials for low-temperature firing which are composite materials, each composed of a ceramic constituent and a glass constituent, and ceramic multilayer substrates using such ceramic materials have been progressively put into practical use.
For example, Patent Document 1 proposes a ceramic raw material composition including: 10 to 45 weight % of a BaO—TiO2—ReO3/2 based ceramic composition represented by xBaO—yTiO2-zReO3/2 (where x, y, and z represent mol %; 8≦x≦18, 52.5≦y≦65, and 20≦z≦40; x+y+z=100; and Re is a rare earth element); 5 to 40 weight % of alumina; and 40 to 65 weight % of a borosilicate glass composition containing 4 to 17.5 weight % of B2O3, 28 to 50 weight % of SiO2, 0 to 20 weight % of Al2O3, and 36 to 50 weight % of MO (where MO is at least one selected from CaO, MgO, SrO, and BaO), in which the total amount of the BaO—TiO2—ReO3/2 based ceramic composition and the alumina is 35 weight % or more.
In Patent Document 1, the borosilicate glass contained in the composition can suppress the shrinkage behavior of the ceramics during firing, and the high glass viscosity can suppress interdiffusion to and from other low-temperature fired substrates. Furthermore, the ceramic composition can provide a ceramic raw material composition which has a high specific dielectric constant ∈r on the order of 15.